The invention relates to a piezoelectric acoustic alarm comprising a resonance chamber with specific resonance frequency and a piezoelement coupled thereto connected via voltage amplifiers and an oscillation means producing a square-wave voltage to a direct current supply.
Sound generators of this type with which an acoustic warning signal is produced on the basis of the oscillations of a piezoelectric crystal or similar material with piezoelectric effects produced by electrical energy in reversal of the piezoelectric effect (electoscriction) have long been known in alarm systems. When an electric voltage is applied to a crystal of this type its shape is altered in order to return to its starting shape when the electric voltage is removed. The application of a pulsed voltage causes rapid oscillation of the crystal and consequently the emission of sound waves.
Acoustic alarms with a piezoelectric transducer naturally requiring a small amount of space are common in various fields of application, such as in error warning devices, fire alarms and smoke detectors, monitors for medical instruments and the like. However, as they do not generate a sufficiently loud sound volume they are limited in their possibilities of use. In many applications, for example in warning devices for persons working under dangerous conditions, such as divers, firemen, miners etc., it is necessary that under certain critical conditions the warning device triggering an acoustic alarm on the one hand does not exceed a specific dimension in terms of size and weight but on the other hand generates an acoustic signal which cannot be missed by the relevant person under the respective conditions.
Furthermore, the energy available and, more precisely in relation to the limited space available and in specific applications the explosion protection is limited.
The object of the invention is therefore to provide a piezoelectric acoustic alarm providing a high sound volume with a small space requirement and low current consumption.
The object is achieved according to the invention by a piezoelectric acoustic alarm designed in accordance with the features of claim 1.
The idea forming the basis of the invention consists in that two piezoelements are arranged opposite one another in a single resonance chamber and are excited by a square-shaped pulsating device voltage in such a way that they move or bend away or toward one another and therefore each oscillate in opposing directions. The opposing oscillation of the two piezoelements is achieved in such a way that a respective square-wave voltage pulsating between zero and a positive value is applied to the one piezoelement and a reversed square-wave voltage alternating between zero and a negative value respectively is applied to the other piezoelement and both square-wave voltages are phase displaced with respect to one another by a pulse length, starting from appropriate control siginals of a microprocessor. According the first piezoelement is bent in one direction upon a voltage pulse while the second piezoelement is bent in the opposing direction upon its reversed voltage pulse in the subsequent current-less phase of the first piezoelement now oscillating back.
Owing to the phase-displaced application of a respective reversed oscillating voltage, oscillations of two piezoelements which are directed toward one another are produced in one and the same resonance chamber, the common sound volume of which oscillations is greater than that from two individual piezoelectric sound generators. In addition, the space requirement and the component and control complexity are lower in the proposal according to the invention and owing to the phase-displaced supply of current the voltage peaks standing in the way of explosion protection regulations are avoided.
Further features and advantageous developments of the invention are found in the sub-claims and the description of a preferred embodiment of a sound generator given by way of example hereinafter.